Modern telecommunications rely heavily upon transmission of light signals across long spans of optical fiber. As the light propagates from a transmitter to a receiver through the optical network, it loses energy (primarily due to light scattering) in the transmission fiber itself and also (by more general loss mechanisms) in the other fiber-optic components of the network. In order to compensate for this energy depletion, the optical power of a signal is repeatedly amplified by optical amplifiers. An erbium-doped fiber amplifier, or EDFA, which amplifies optical signals in the wavelength range from about 1520 nm to 1620 nm, has become an integral part of most modern optical networks. This is discussed, for example, in the new volumes (IIIA and IIIB) of the series “Optical Fiber Telecommunications”, edited by I. P. Kaminow and T. L. Koch, Academic Press (1997). The increasingly common use of EDFA's in this context now often leads to a situation where it is desirable to have more than one amplifier at a single location in the network. It has therefore been proposed to use in such instances special modules incorporating multiple separate amplifiers, or amplifier arrays. Examples of arrayed rare-earth doped amplifiers are described in “Planar Er— and Yb-doped amplifiers and lasers” by Balslev, S.; Dyngaard, M.; Feuchter, T.; Guldberg-Kjaer, S.; Hubner, J.; Jensen, C.; Shen, Y.; Thomsen, C. L.; Zauner, D. Applied Physics B v.73(5–6), pp.435–438, 2001, and in “New WDM amplifier cascade for improved performance in wavelength-routed optical transport networks” by Olivares, R.; Baroni, S.; Di Pasquale, F.; Bayvel, P.; Anibal Fernandez, F. Optical Fiber Technology v.5(1), pp.62–74, 1999.
An erbium-doped waveguide amplifier, or EDWA (a recent example of which has been described in U.S. Pat. No. 6,157,765 by A. J. Bruce and J. Shmulovich), has properties similar to those of an EDFA, and therefore its functionality is equally important. However, unlike EDFA's, EDWA's are waveguides manufactured on planar substrates using glass hosts that may differ dramatically in composition from those used in EDFA's. Although generally somewhat less efficient than EDFA's, in many instances EDWA's have advantages compared to their fiber analogs: for example, a packaged EDWA chip has much smaller size than a packaged EDFA. Moreover, it is natural and straightforward to integrate an EDWA with other passive or active optical components on the same planar substrate, an assembly that is impossible with EDFA's. Also, in some instances the integrated module may be able to perform functions that are not achievable by its modular fiber-optic analog.
Typically, at least two kinds of optical energies are present in EDFA's and EDWA's: first one is that of one or more signals with wavelengths from around 1520 to 1620 nm, the second one is that of one or more optical pumps with wavelengths around 980 nm and 1480 nm. The purpose of the pump light is to deliver energy to Er ions in an EDFA and excite them; a part of that energy is subsequently transferred to the signal light resulting in its amplification. Amplifier arrays become especially attractive when the number of required pump sources is less than the number of individual amplifiers. For instance, if a single pump laser is used to simultaneously provide pump power for four separate amplifiers, it could potentially result in four-fold savings in pump cost. U.S. Pat. No. 6,008,934 by Fatehi et al. describes a module incorporating several essentially independent amplifiers and pump power splitters. The latter splitters provide fixed and equal pump power distribution among all the amplifiers, thereby making them interdependent and prohibiting independent gain control in separate amplifiers. Therefore, the use of such a module is limited only to narrow band, single channel amplification. In order for this module to be useful in broad band applications utilizing several wavelength-multiplexed channels, each amplifier has to be provided with a means of independent gain control. Typically, this is accomplished by varying the pump power, which so far could only be achieved by providing each amplifier in the amplifier array with a separate pump laser.